The fabrication of integrated circuits is an extremely complex process that may involve hundreds of individual operations. The figure of merit of a semiconductor manufacturing facility is the sort yield obtained by electrically probing the completed devices on wafers. Once the front-end processing of the wafers is completed, the dies on the wafers are tested before they are sent to die preparation. Dies on the wafers which fail the testing are considered faulty and are discarded or scrapped. The sort yield is the proportion of devices on the wafer found to perform properly.
Since the fabrication of wafers take weeks in cycle time and a single processed wafer can have a value of tens of thousands of dollars, it is advantageous to detect problems early in the process in order to correct them, rather than wait to test the wafers once the front-end processing is complete. In order to minimize the at-risk product at final test, inline metrology and inspection steps, that is, steps performed in conjunction (“inline”) with the fabrication process, as opposed to at the end of the process, are used to monitor wafer manufacturing throughout the fabrication sequence. Metrology and inspection technology includes defect inspection (to identify defects) and defect review (to help extract information from the identified defects to improve the design or alter the fabrication process in an effort to enhance the sort yield). The defects detected by inspection tools are referred to as ‘visual defects.’ Wafer test metrology equipment is used to verify that the wafers have not been damaged by previous processing steps. If the number of dies, on a wafer that measure as fails, exceeds a predetermined threshold, the wafer is scrapped rather than investing in further processing. Thus, it has been a goal in the industry to detect visual defects and to review the defects to determine the cause as early as possible.
Fabrication plants must capture a wide range of problems on “patterned” wafers (i.e., wafers having the circuit patterns imprinted on them), such as physical defects and electrical defects, which can ruin an entire wafer rather than just a die. As wafers move between processing steps, defect inspection systems identify the location of defects, such as particles, open lines, shorts between lines, or other problems, on the patterned wafers.
Using inspection systems to detect defects is only the first step in managing defectivity. Manufacturers must also be able to sample the defects to review and identify their causes quickly. The sampling of defects may be performed by defect review systems which may utilize Scanning Electron Microscopy (SEM) technology. SEM uses an electron beam to image and measure features on a semiconductor wafer at a much higher resolution than images captured by optical microscopes that may be used during defect inspection.
In many cases, inspection systems identify hundreds of defects for each wafer. However, not all of the identified defects are of equal importance to the device yield. Thus, not all of the defects warrant further review by defect review systems. Inspection equipment, especially in recent technologies—where the optical inspection is pushed to it's sensitivity limit in order to detect sub-micron sized defects—is often plagued with many false alarms or nuisance defects, as they are known, which serve to frustrate any attempts to reliably observe true defects or sources of defects. The problem with current review procedures is that defects are reviewed, regardless of whether they are nuisance defects. The result is an inefficient review process that may produce an inaccurate yield prediction.